1. Field of the Invention
The present invention relates generally to the heat treating of metal such as coils of steel in a process known as annealing. More particularly, the present invention relates to the provision of and the use, in conjunction with the operation of a single-stack annealing furnace, of a set of novel elongate modules of compressed, reinforced fiber refractory material to form an inner seal of the furnace, with the inner seal preferably including upper and lower blankets of refractory fiber material that sandwich therebetween a tightly packed end-to-end arrangement of the modules that, together with refractory fiber spacer blocks that preferably are utilized to separate adjacent pairs of the modules, circumferentially fill an upwardly opening seal positioning trough that has a cross section that narrows with depth, with the trough preferably being defined between inner and outer members of a novel set of cast refractory segments that form a rigid ceramic refractory base of the furnace. The cast refractory segments and the inner seal modules may be assembled on-site, or at a remote location for transport to and installation as a unit at a furnace site. The invention extends to features of the cast refractory and fiber seal base components, to features of furnace bases assembled from these novel components, to tools that preferably are used in installing, maintaining and repairing fiber seals in annealing furnace bases, and to methods of fabrication, assembly, use, maintenance, repair and replacement.
2. Prior Art
In a single-stack annealing furnace, a fixed base typically is used to centrally support a charge of metal that is to be treated by subjecting the charge to an annealing process which typically includes a lengthy, controlled heating and controlled cool-down process in the environment of a treatment chamber wherein inert gas is circulated. The treatment chamber is defined in large measure by an open-bottom, tank-like inner enclosure of the furnace that is lowered into place once the charge of metal has been positioned centrally atop an inner part of the base. The inner enclosure has a bottom rim that compressively engages an inner seal of the base which extends perimetrically about the inner part of the base. Spaced outwardly from the inner seal is an outer seal that is engaged by an outer enclosure of the furnace that is lowered into seated engagement with the outer seal to heat a furnace chamber within which the inner enclosure is contained, which, in turn, transfers heat energy into the controlled environment of the treatment chamber.
The inner seal typically is called upon not only to seal the treatment chamber 1) against the loss of its controlled gas atmosphere and 2) against contamination of the controlled atmosphere by leakage of ambient air into the treatment chamber, but also to physically support much, if not all, of the weight of the lowered-in-place inner enclosure, the bottom rim of which is seated atop the inner seal once the inner enclosure has been lowered into place. In contrast, the while the outer seal typically is called upon 1) to prevent unwanted loss of heat energy from the furnace chamber and 2) to prevent entry into the furnace chamber of ambient air, the outer seal is seldom required to physically support much, if any, of the weight of the lowered-in-place outer enclosure of the furnace.
Sand has been widely used to form one or both of the inner and outer seals of annealing furnaces. While sand is desirable from the viewpoints 1) of being relatively inexpensive and 2) of being capable (if the sand happens to be distributed in a void-free and uniform manner beneath and along the entire perimeter of a depending rim of a furnace enclosure) to provide a reasonably effective seal, the use of sand in the highly active environment of a steel production facility is quite undesirable due to the fact that grains of sand are small and lightweight in character, and tend to spread themselves about the facility causing severe problems of product contamination.
Unacceptable sand contamination of steel product can result from a single grain of sand being moved out of either of an inner seal trough or an outer seal trough of an annealing furnace. For example, if a grain of sand is lifted above an annealing furnace base during the raising of one of the inner or outer enclosures of the furnace, and if the sand grain falls from the raised enclosure to become lodged in one of the many narrow spaces that may be present among adjacent wraps of a coil of steel, the errant sand grain probably will be pressed into the steel when the steel passes through the rolls of a temper mill, thereby causing an unacceptable product imperfection that, if found to be present very frequently in the output of a mill, may cause customers to purchase elsewhere.
In an effort to eliminate the use of sand seals in annealing furnaces, a wide variety of proposals have been made, some of which have made use of fiber refractory materials of various forms that are laid in place in upwardly opening seal positioning grooves. While sand-substitute fiber seal proposals have, to some degree, been found to serve adequately to provide non-load-bearing outer seals of annealing furnaces, fiber seal proposals for use as load-bearing inner seals have inherently encountered a variety of drawbacks, chief among which has been their unduly high cost of use. Inner seals formed from refractory fiber have tended to be easily damaged during normal service use, have tended to be easily crushed under the weight of the inner enclosures that they must support, have tended to quickly lose their resilience or to otherwise quickly fail to provide gas impermeable barriers, and have, for these and other reasons, tended to require frequent replacement at unacceptably high cost.
Thus, while the desirability of utilizing refractory fiber materials to form outer and inner seals of annealing furnaces has been recognized, a problem that has been encountered in efforts to provide sand-substitute, fiber-type inner seals--a long-standing problem that has tended to defy the finding of a suitable solution--has been the combined need to provide a fiber-type inner seal structure that will remain sufficiently resilient over a suitably lengthy service life to ensure that a gas-impervious seal of good integrity is reliably maintained, while, at the same time, offering sufficient crush resistance and structural integrity to suitably support the weight of an inner furnace enclosure.
While the desirability of utilizing costly, high technology castable refractory materials to form bases of annealing furnaces also has been recognized, efforts that have been made to mold-form these cantankerous materials in situ at the sites of an annealing furnaces have not met with good success. The type of cast refractory materials that are available at present-day that can be mold-formed to provide rigid ceramic structures that will withstand use in a steel production facility where temperatures are repeatedly cycled between ambient temperature and temperatures of about 1500 degrees Fahrenheit (and higher) are low cement containing mixtures that include about 45 to about 47 percent alumina (Al.sub.2 O.sub.3), about 45 to 47 percent silica (SiO.sub.2), and that contain about 2 percent, by weight, of thin stainless steel needles (that typically are about an inch in length and are included to provide strength and reinforcement to the resulting product)--which are mixed with a sufficiently small quantity of water to barely bring the material to a dry granular consistency that can be fed into a mold without causing a cloud of dust to arise as the mix is fed into the mold, and which require the presence of power-induced mold vibration in order to ensure that the material is properly distributed throughout the mold to form a mixture of even consistency that can be cured to form a strong, temperature-cycle-resistant product.
To achieve the uniformity and high density of refractory material that is needed in the resulting product, it is important that the water content of a cast refractory mix be carefully controlled and kept to a minimum, that the vibration that is applied to the mold be sufficiently powerful to thoroughly vibrate the mold for substantially the entire period of time that the mold is being filled, and that the newly molded product be carefully cured in a temperature controlled environment--little, if any, of which tends to be properly carried out if what one tries to do is to mold an annealing furnace base in situ at a furnace site.
Forming cast refractory members to provide components of annealing furnace bases has even proved to be a difficult undertaking to carry out in a specialized cast refractory production facility due to the enormous size and weight of the members that need to be formed, and due to the massive amounts of cast refractory material that need to be aggressively vibrated into place in massive molds or forms. If base components are made that are too small in size, the number of components that must be installed, the nature of the mistakes that can be made in installing components, and problems of component breakage unduly complicate the work of effecting full-base replacements. On the other hand, the larger that components are made, the heavier they are to move, the more difficult they are to properly position, and the less forgiving they are of accommodating dimensional irregularities that are encountered to some degree in almost every base replacement endeavor. Finding a "right approach" to the sizing and shaping of remote-facility-molded cast refractory segments for annealing furnace bases has proved to be elusive.
While efforts have been made to mold whole furnace bases and base portions off-site at facilities that specialize in the fabrication of mold-formed castable refractory structures by mold-forming castable refractory materials, such efforts have met with very differing degrees of success depending often on the extent to which success can be had in transporting the resulting structures to, and in crane-lifting such structures into place at, a furnace site. Trying to use lift truck forks to maneuver cast refractory structures, and trying to lift and position cast refractory structures utilizing crane-supported cables that wrap about or otherwise engage outer surfaces of the newly molded cast refractory structures tends to cause unacceptable chipping, cracking and breakage. Moreover, incorrectly stressing or inadequately supporting these massively heavy cast structures during transport or during lifting or positioning, can easily cause the newly cast structures to break apart under their own weight.
Thus, while the desirability of forming cast refractory annealing furnace bases has been recognized, the need for a practical method that will actually enable cast refractory bases of high structural integrity and offering reliably good performance characteristics to be provided and installed with excellent consistency has gone unfulfilled.
Another problem that has been encountered with annealing furnace bases is the severe warping and cracking of, and hence the need for frequent replacement of, structural steel that typically is welded in place in the vicinities of the inner or outer seals of the furnace. Inner walls of the outer seal troughs of annealing furnaces have, for example, typically been formed from structural steel that is held in place by virtue of being welded to an underlying base support structure of the furnace; and this structural steel often is found to warp severely and to break loose from its welds long before the service life of an adjacent cast ceramic base has come to a close.
Because structural steel does not fare well when subjected to repeated cycling between ambient temperature and elevated temperatures of up to 1500 degrees Fahrenheit (and higher), and because welds of structural steel also perform poorly when subjected to repeated temperature cycles of this type, it has been recognized as being desirable to eliminate or minimize the use of structural steel and structural steel welds in the vicinities of the inner and outer seals of annealing furnaces. However, it has been widely accepted that cast refractory materials do not have sufficient strength and sufficient impact resistance to be used either in place of such structural steel or in reinforcing welded steel structures that may need to be used to define the outer seal trough of an annealing furnace. Some of the features of the present invention break new ground in successfully employing cast refractory materials in unconventional uses of this type.
Because the base structures of annealing furnaces are subjected to repeated cycles of high temperature heating followed by cooling, and because heavy loads are imposed on these structures as both massive charges of metal and heavy furnace enclosures are moved into and out of position, annealing furnace base structures need to be serviced and repaired frequently, and replaced regularly as a part of scheduled programs of maintenance--which is true regardless of the character of the materials from which the bases are formed. Far too much "down time" presently is needed to maintain, repair and replace the bases of annealing furnaces. Bases are needed, and base maintenance, repair and replacement tools and techniques are needed, that will permit the maintenance, repair and replacement of annealing furnace bases to be carried out while requiring much less "down time."
3. The Referenced Cases
The referenced Cast Refractory Segment Cases disclose a number of annealing furnace base segment configurations that can be used in conjunction with features of the preferred practice of the present invention. The referenced Companion Case discloses a preferred manner in which features of the present invention, together with other invention features, are put to use in the environment of a plural stack annealing furnace. Due to the related nature of these referenced cases, their disclosures are incorporated herein, by reference.